Allow pybind11::arg to have 0, false, or "" as default values.

Skip FindPythonLibsNew if PYTHONLIBS_FOUND is defined (fix #236)

Also:
- Update documented result variables
- Remove long-ago-deprecated variable

Check the number of named arguments at compile time

Syntax sugar: user-defined literals for arg and str

A few smaller fixes and a test output improvement

It used to pass anyway because the expected output was identical
to the Python set.

- Conversion warning on clang: 'long' to 'size_t'
- Unused variable warning on MSVC

Fix Cygwin support